Liquid-Liquid Extraction is an important procedure for the recovery of valuable species or for the purification of streams. It plays a central role in numerous processes in the chemical, hydrometallurgical, biotechnological and pharmaceutical industries. When applicable, it is preferred over other separation procedures because it uses relatively little energy and does not expose the processed materials to extreme temperatures. Conventional Liquid-Liquid Extraction, as performed today, consists of a four step procedure: Firstly, the donor liquid solution is brought in close contact with a selected immiscible extractant having a density that is different from that of both the donor and the recipient solutions, at conditions (compositions or temperatures or pH) that promote the transfer of the species from the donor solution to the extractant. This is achieved by creating a mixture of droplets of the extractant in the solution or of droplets of the solution in the extractant. Secondly, after having provided sufficient time for the transfer to take place, the extractant phase is separated from the solution phase by gravitation and/or centrifugation. Thirdly, the now loaded extractant is brought in close contact with the recipient solution at new conditions (compositions or temperatures or pH) that are conducive to the transfer of the species from the extractant to the recipient solution, by again creating a mixture of droplets of the extractant in the solution or of droplets of the solution in the extractant. Finally, after having provided sufficient time for the transfer to take place, the extractant phase is separated from the solution phase by gravitation and/or centrifugation. Then, the extractant having been freed of the extracted species is mostly recycled, directly or following purification, for reuse in the first step. The first two steps are named the extraction stage and the second pair of steps is named the back-extraction stage. Thus, each extraction (or back-extraction) stage consists of a mixing+phase separation set to which two feed streams are fed, one a heavier phase, typically aqueous and the other a lighter phase, typically an oil and generating two streams, again one aqueous and one oily. It sometimes happens that the extractant used, in addition to its affinity to the species of interest in the donor solution also has affinity to additional species present therein. When interested to obtain the extracted species in distinct recipient solutions, one or more additional back-extraction stages may be added to the cycle that will then consist of an extraction stage followed by more than one back-extraction stages. Since the case involving several distinct separated species, while adding to the complexity of the process, does not affect the principle of the operation, our discussion will center for simplicity, on a cycle comprising a single extraction followed by a single back-extraction stage. As the amount of species transferred in a single extraction or back-extraction stage is usually limited, stages of extraction and stages of back-extraction are mostly organized in series to form a train of extraction stages and trains of back-extraction stages in each of which the solution flows in a counter-current or a co-current direction to the extractant. Thus, a larger amount of the species can be transferred from the donor solution to the extractant and then from the extractant to the recipient solution thereby enriching the recipient solution in the transferred species producing an enriched recipient solution while impoverishing the donor solution in the transferred species producing a raffinate solution. The physical implementation of a train of extraction or back-extraction stages takes mostly the form of a battery of Mixer-Settlers or of one of several types of Column Extractors. The Liquid-Liquid Extraction procedure is sometimes included within a broader process to perform a separation task as a part of a wider scheme (e.g. a reaction). When the process utilizing Liquid-Liquid Extraction is designed for the sole purpose of component separation, it is called an extraction process. A typical extraction process may include, in addition to one or more extraction trains, other processing units to effect washing, stripping, distillation, etc. in a network of streams. The product of interest in an extraction process may either be the raffinate solution, or the enriched recipient solution, or both.
The design of an extraction plant is a complex task. Given a separation objective, a suitable extractant must first be identified and then a processing path can be delineated. Given the non-negligible cost of most extractants, a Liquid-Liquid Extraction plant will seek to recycle the extractant in a closed cycle. When the extractant regeneration is all done in situ, the plant is characterized as a closed-extractant-cycle plant. Mutual immiscibility of the phases is a basic requirement in all Liquid-Liquid Extraction processes. The majority of applications use an oily extractant to process aqueous solutions but the other way around is just as valid.
Numerous attempts have been made to improve the performance of the equipment used to implement Liquid-Liquid Extraction where two liquid streams, one aqueous and the other oil, are brought into contact in Mixer-Settlers or Extraction Columns. To mention just a few examples, in U.S. Pat. No. 3,914,175 Kunz proposes a modification of the settler to facilitate the separation of the phases, in U.S. Pat. No. 4,268,484 Gavin addresses the arrangement of the mixing and settling chambers in a Mixer-Settler plant, in U.S. Pat. No. 4,292,277 Bonney et al. suggest a rearrangement of the flows within a Mixer-Settler plant, in U.S. Pat. No. 4,545,901 Schneider addresses the energy invested in agitation, in U.S. Pat. No. 4,200,525 Karr suggests the contacting of the phases in a reciprocating plate Extraction Column on the basis of a specific equation, in U.S. Pat. No. 4,609,457 Kilroy suggests an operation and control method for the Extraction Column. A common feature of all those patents is that they all persist in the paradigm stating that Liquid-Liquid Extraction is carried out by mixing intimately two streams, one aqueous and the other oily and then rely on a density difference to separate them. Also, following this paradigm, when it is needed to transfer a solute from one aqueous solution to another, the extraction must be applied twice in two distinct apparatus, in the first the solute is transferred from one aqueous solution to the oily phase and then in the second it is transferred from the oily phase to the other aqueous solution.
The first attempt to break away from this paradigm was the introduction of the Supported Liquid Membrane (SLM) concept where a solute is transferred directly from one aqueous solution to another by permeating through a supported liquid membrane. In U.S. Pat. No. 4,851,124 Vandegrift, et al. propose the immobilization of the oily component on a membrane thereby forming a supported liquid membrane that will then separate two aqueous solutions, causing a solute to permeate through the membrane from one aqueous solution to the other. Thereafter, the application of Supported Liquid Membranes for liquid extraction reappears as the subject of numerous additional US patent references. Still, with all this activity around SLMs, actual practical application of the SLM method to perform industrial separations in bulk is scarce, mainly because of the difficulty in maintaining, stably and over a significant time span, a whole and continuous liquid membrane that is thin enough to provide a meaningful mass transfer rate. In view of this difficulty with the SLM approach, an alternative approach, making use of membrane permeation, but splitting the extraction and back-extraction into two distinct steps, was developed. In U.S. Pat. Nos. 4,789,468 and 4,997,569 Sirkar describes an Immobilized-Interface Solute-Transfer apparatus where two streams, one an extractant and the other a solution pass through two adjacent compartments separated by a membrane. A solute permeates from the solution through the membrane to the extractant or the other way around. Stability of the operation is facilitated by controlling the differential pressure across the membrane. Alternatively, Sirkar suggests a three-compartment option, combining the functions of two two-compartment units into an extraction/back-extraction scheme. The extractant in a central compartment is separated from two solutions flowing in two additional compartments by two membranes. This causes a solute in the feed solution stream to permeate through a first membrane into the extractant and then migrate from the extractant through the second membrane to the second solution stream. Here again stability of the operation is facilitated by controlling the differential pressure across each of the two membranes. Sirkar's method prefers the membrane to be in the form of a hollow fiber membrane, which has led it to be named in consequent publications, Hollow-Fiber Contained Liquid Membrane, in short HFCLM. The three-compartment HFCLM can be viewed as an extended SLM where the single SLM membrane has been replaced by a double membrane enclosing a body of extractant, imparting it with added stability at the expense of an increased resistance to mass transfer. Mass transfer is indeed a major consideration in all membrane assisted liquid extraction and it has consequently become a focus of interest in the published literature. With the ultimate goal of creating a closed extractant cycle extraction/back-extraction process, two separate two-compartment units, one for extraction and the other for back-extraction is evidently more flexible than using a single three-compartment unit. Like the SLM method before it, the HFCLM approach circumvents the problems caused by phase dispersion and then separation plaguing conventional Liquid-Liquid Extraction. However, two major problems afflict all membrane based extraction processes: The first relates to the stability of membrane, namely the prevention of one phase to leak through the membrane to another phase, thereby degrading its function. Sirkar addresses this problem by controlling the differential pressure across the membrane. The second problem concerns the rate of mass transfer which is hindered by a series of resistances, three resistances in SLM and six to seven resistances in series for HFCLM, the sum of which naturally results in a significant overall resistance to mass transfer with consequent limited throughput. The concept of Liquid-Liquid Extraction in a thin extractant layer was first mentioned in a paper (Dolev, Kehat and Lavie, Ind. Eng. Chem. Res. 1999, 38, 1618-1624). It advanced the hypothesis that a bed of extractant-coated pellets could be used to form the equivalent of an adsorbent bed in a temperature swing process, causing a feed solution of constant composition and periodic temperature to yield a stream varying in composition and temperature over time. The results, while indicating some extraction effect, lacked practical significance because it failed to recognize the central role played by the extractant to solution ratio or the relative thickness of the phases brought into contact, it did not offer a practical implementation relevant to the way a Liquid-Liquid Extraction cycle operates, and offered no solution to the instability of the thin extractant layer, which caused the extraction effect to deteriorate within a limited number of cycles. Numerous studies concerning the use of micro-porous membranes to perform various fluid separation tasks have recently been published. Gas-gas separations and gas-liquid separations have been successful to the point of establishing numerous industrial applications. Liquid-Liquid Extraction through micro-porous membranes has not been as fortunate so far. Nevertheless, considerable understanding of the problems afflicting membrane separation of liquids has accumulated. For example, Serengupta et al. (AIChE J. 1988, 34 p. 1698 and Sep. Sci. Technol. 1988, 23, p. 1735) have found that using a pure extractant, rather than a modified and diluted one, considerably improves membrane stability. Numerous studies address the resistances to mass transfer in HFCLM, tabulating values for the individual resistances pertaining to various fluids.